The practice of capturing color images with digital imaging devices or systems, such as a digital camera or color scanner, is widely spreading. Currently, these digital devices include a charge-coupled device or complementary metal-oxide-semiconductor (CCD/CMOS) sensor array with a set of filters before it. Ideally, these digital imaging devices capture color images in a substantially accurate and aesthetically pleasing manner.
There are many criteria which are utilized in the design and production of color imaging devices. An important consideration is the ability of the device to deliver color signals which can be used to create high quality color reproductions. To quantify the color quality capability of such devices, it is important to determine how the device's response to color stimuli corresponds to that of a human. Through psychophysical experiments, the Commission Internationale de l'Eclairage (CIE) has generated a standard set of color matching functions for the standard observer, representing the average human visual response to color. As the human eye has three types of cones, each with a different spectral sensitivity signature, there are three standard function specified by the CIE. Accordingly, most color imaging devices are set up with three channels and the spectral sensitivities in these imaging devices are initially designed to attempt to mimic the human visual system.
The spectral sensitivity functions for the color imaging channels in the device should satisfy the Luther condition, that is the channel spectral sensitivities need not be exact duplicates of the CIE color-matching functions described above, but need to be a nonsingular transformation of them. In practice, it is not always possible to manufacture filters for imaging devices that satisfy the Luther condition, often due to the physical limitations of fabricating process. Measurement noise also plays an important role and will degrade the color accuracy even when spectral sensitivity curves fulfill the Luther condition.
Accordingly, it is desirable to have spectral sensitivities which satisfy certain criteria, such as mimicking the human visual system. Unfortunately, it has been difficult to find systems and methods which can accurately and efficiently optimizes spectral sensitivities in an imaging device.